With a drastic increase in network traffic, wavelength division multiplexing technologies that enable large volume data transmission have become widely used. Also, there is a demand to transmit optical signals over a long distance without converting them into electric signals along the transmission path. However, in general, the speed of light transmitted through an optical fiber varies depending on the wavelength of the light. Therefore, even when multiple lights are emitted at the same time from a transmitting end, the lights arrive at the same receiving end at different timings depending on their wavelengths. This phenomenon is called wavelength dispersion.
Generally, signals are modulated before transmission. A modulated optical signal naturally has various widths in frequency. In other words, a modulated optical signal is composed of lights with different wavelengths. Accordingly, the waveform of a received optical signal tends to be distorted due to the wavelength dispersion and if excessive distortion occurs, it becomes difficult to correctly receive information. To compensate for the wavelength dispersion and thereby prevent this problem, when an optical signal is received at each node, the optical signal is processed by a dispersion compensation module (DCM) having wavelength dispersion characteristics that are opposite to those of optical fibers constituting the transmission path. This configuration makes it possible to transmit an optical signal over a long distance without distorting the waveform of the optical signal.
However, since the amount of wavelength dispersion increases in proportion to the distance and different types of optical fibers have different characteristics, it is necessary to use different DCMs depending on distances and the types of optical fibers. Here, since an inexpensive DCM is typically composed of passive components such as optical fibers, one type of DCM generally has a fixed characteristic. Therefore, it is necessary to determine in advance the locations and the characteristics of DCMs to be provided on the network. Determining the locations and the characteristics of DCMs on the network is called dispersion compensation design.
Here, the amount of wavelength dispersion compensation provided by a DCM preferably matches the amount of wavelength dispersion caused by an optical fiber. However, since DCMs are typically composed of passive optical components such as dispersion compensation optical fibers to reduce power consumption, it is not practical from a cost standpoint to produce DCMs suitable for respective optical fibers. For this reason, a DCM is generally configured to provide various amounts of wavelength dispersion compensation that are represented by graduated (or discrete) values and can be selected from a menu. In this case, even if an amount of wavelength dispersion compensation closest to the amount of wavelength dispersion compensation necessary for an optical fiber is selected, the selected amount of wavelength dispersion compensation is insufficient or excessive (this is called a compensation error) for the optical fiber by about one half of the interval between the graduated values.
FIG. 1 illustrates exemplary dispersion compensation results, and FIG. 2 illustrates an exemplary compensation amount menu of a DCM where dispersion compensation amounts are provided at intervals (or steps) of 100 ps/nm. In FIG. 1, N1 through N6 indicate nodes, and lines connecting the adjacent nodes indicate optical fibers and are called spans in the descriptions below. Figures under each span indicate, from the top to the bottom, the amount of wavelength dispersion (hereafter called the amount of dispersion) of the corresponding optical fiber, the amount of dispersion compensation, and the amount of dispersion after compensation. “Excessive compensation” in the bracket indicates that the amount of dispersion compensation is greater than the amount of dispersion of the optical fiber, and “insufficient compensation” in the bracket indicates that the amount of dispersion compensation is less than the amount of dispersion of the optical fiber. For example, the amount of dispersion of the optical fiber between the nodes N1 and N2 is 253 ps/nm and the closest amount of dispersion compensation in the menu of FIG. 2 is 300 ps/nm. In this case, the amount of dispersion compensation is excessive by 47 ps/nm.
Thus, with the compensation amount menu of FIG. 2 where the amounts of dispersion compensation are provided at the intervals of 100 ps/nm, a compensation error of ±50 ps/nm may occur. In a network of recent years where optical fibers are connected to each other like a mesh, optical fibers for which the amount of dispersion compensation is insufficient and optical fibers for which the amount of dispersion compensation is excessive tend to be distributed nearly at random.
FIG. 3 illustrates an exemplary network including seven nodes. The network of FIG. 3 includes nodes N1 through N7. A first figure below each span between the adjacent nodes indicates the amount of dispersion of the corresponding optical fiber and a second figure below the first figure indicates the amount of dispersion compensation.
FIG. 4 is a table illustrating exemplary tolerable ranges of cumulative dispersion. The tolerable range varies depending on the distance. In FIG. 4, the distance is represented by the number of spans that have substantially the same length. FIG. 5 is a table illustrating cumulative dispersion from a start node to an end node. When the node N1 is the start node and the node N2 is the end node, the cumulative dispersion of the path N1-N2 is 5 ps/nm that is obtained by subtracting the amount of dispersion compensation from the amount of dispersion of the optical fiber corresponding to the span N1-N2. In this case, the number of spans in the path N1-N2 is one. According to FIG. 4, when the number of spans is one, the tolerable range of cumulative dispersion is between 70 ps/nm and −30 ps/nm. Accordingly, 5 ps/nm is within the tolerable range and the path N1-N2 is capable of transmission (i.e., a signal or information can be transmitted correctly through the path N1-N2). When the node N1 is the start node and the node N7 is the end node, the path N1-N7 includes six spans (N1-N2, N2-N3, N3-N4, N4-N5, N5-N6, and N6-N7). Also in this case, since the cumulative dispersion of the path N1-N7 is 32 ps/nm and the tolerable range is between 35 ps/nm and 5 ps/nm, the path N1-N7 is capable of transmission.
Meanwhile, in the case of a path N1-N6 including five spans, the cumulative dispersion is −3 ps/nm and the tolerable range is between 42 ps/nm and −2 ps/nm. Therefore, the path N1-N6 is not capable of transmission. In this example, although signal transmission is not possible through the path N1-N6, signal transmission is possible through the path N1-N7 that is longer than the path N1-N6.
FIG. 6 is a flowchart illustrating a related-art dispersion compensation design process. As illustrated in FIG. 6, an arrangement of DCMs is generated in step S1 and whether all paths are capable of transmission is determined in step S2. If all the paths are capable of transmission, the process is completed. Meanwhile, if one (or more) of the paths is not capable of transmission (hereafter, a path not capable of transmission is called “incapable” path), in step S3, a regenerative repeater(s) is added in the middle of the incapable path.
FIG. 7 is a flowchart illustrating another related-art dispersion compensation design process. As illustrated in FIG. 7, an arrangement of DCMs is generated in step S5 and whether all paths are capable of transmission is determined in step S6. If all the paths are capable of transmission, the process is completed. Meanwhile, if one (or more) of the paths is not capable of transmission, a constraint that prevents use of the combination of the amounts of dispersion compensation currently applied to the incapable path is added and step S5 is repeated.
Japanese Laid-Open Patent Publication No. 2010-098559, for example, discloses a technology for securing the quality of optical paths. In the disclosed technology, a remaining wavelength dispersion target value (RDT) is determined for a first wavelength path having higher priority and a remaining wavelength dispersion tolerable range (RDR) is determined for a second wavelength path with lower priority, and the amounts of compensation of DCMs are determined such that the difference between the RDT and the remaining wavelength dispersion value of the first wavelength path is minimized and the remaining wavelength dispersion value of the second wavelength path falls within the RDR.
Also, WO 2004/088921, for example, discloses a network design technology where a network including multiple channels and paths is divided into partial linear networks each including a terminal node and a branch node. In the disclosed technology, an optical amplifier and a regenerative repeater are assigned to each partial linear network based on the performance of the optical fiber between the nodes, specific paths are formed by combining the partial linear networks, and channel termination devices assigned to the branch nodes are removed based on the signal performance of the specific paths.
Further, Japanese Laid-open Patent Publication No. 2006-042279, for example, discloses a method for determining optimum arrangement (placement or layout) of regenerative repeaters 3R to secure the signal quality of paths in an optical network. In the disclosed method, a network is divided into multiple regenerative repeating sections 3RS each having the regenerative repeaters 3R at both ends, optical amplifiers and optical add/drop multiplexers (OADM) are placed at nodes in the regenerative repeating sections 3RS, multiple assumed paths that are assumed as a result of placing the optical amplifiers and the OADMs are determined, whether the assumed paths are capable of transmission is determined, and the determination results and the assumed paths are displayed to allow the user to reconfigure the network.